The goal of this research is to elucidate the structural and mechanistic details of hydroporphyrin containing enzymes. Interest is centered upon nitrite and sulfite reductases, two related classes of enzymes which carry out the unusual six-electron reductions of their substrates. These enzymes minimally contain siroheme, an iron-isobacteriochlorin complex, and a ferredoxin-like Fe4S4 center as prosthetic groups. Synthetic analogues (models) of these prosthetic groups will be prepared by chemical means. The structure, properties, and reactivity of these analogues will investigated in order to gain insight to the function and environment of the prosthetic groups in the enzymes. The as yet unknown complexes containing a heme group linked to an Fe4S4 center will be prepared. The active sites have been proposed to have this type of structure. Investigation of the analogues will provide evidence to determine whether this structure type is present in the enzymes, will aid in identification of the mode of linkage, and will elucidate its properties and functions. Aspects of the structure and reactivity of siroheme analogues will be compared with that of other hemes. Difference may be suggestive of important mechanistic features of the enzymes. The interaction of nitrite and sulfite with siroheme analogues and subsequent reduction to products will be investigated.